A solar term (or jiéqì, ) is any of twenty-four moments in traditional Chinese lunisolar calendars that matches a particular astronomical event and signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by Chinese lunisolar calendar to stay synchronized with the seasons, which is crucial for agrarian societies. The solar terms are also used to calculate intercalary months; which month is repeated depends on the position of the sun at the time.

According to the Book of Documents (書經; Shūjīng), the first determined term was Dongzhi (Winter Solstice) by Dan, the Duke of Zhou (周公旦; Zhōu Gōng Dàn), while he was trying to locate the geological center of the Western Zhou dynasty, by measuring the length of the sun's shadow on an ancient type of sundial called  (). Then four terms of seasons were set, which were soon evolved as eight terms; not until the Taichu Calendar of 104 BC were all twenty-four solar terms officially included in the Chinese calendar.

Because the Sun's speed along the ecliptic varies depending on the Earth-Sun distance, the number of days that it takes the Sun to travel between each pair of solar terms varies slightly throughout the year, but it is always between 15 and 16 days. Each solar term is divided into three (), so there are 72 pentads in a year, consisting of five, rarely six, days. Most of them are named after phenological (biological or botanical) phenomena corresponding to the pentad.

Solar terms originated in China, then spread to Korea, Vietnam, and Japan, countries in the East Asian cultural sphere. Although each term was named based on the seasonal changes of climate in the North China Plain, peoples living in the different climates still use it without changes. This is exhibited by the fact that traditional Chinese characters for most of the solar terms are identical.

On December 1, 2016, the solar terms were listed by UNESCO as an Intangible Cultural Heritage.

Numbering

The solar terms used to mark the midpoint of the month (marked with "Z", for ) are considered the major terms, while the solar terms used to mark the start of the month (marked with "J", for ) are deemed minor. The year starts with Lichun (J1) and ends with Dahan (Z12).

Multilingual list

Chinese mnemonic song

The "Song of Solar Terms" () is used to ease the memorization of jieqi:

1. Traditional Chinese:

1. Simplified Chinese:

1. Pinyin

The first four lines provides a concise version of the names of the 24 jieqi. The last four lines provide some rules of thumb about the Gregorian dates of jieqi, namely:

• Two jieqi per month;

• Gregorian dates are off by one or two days at most;

• In the first half of the year, jieqi happens around the 6th and 21st day of each (Gregorian) month;

• In the second half of the year, jieqi happens around the 8th and 23rd day of each (Gregorian) month.

Determination

The modern definition using ecliptic longitudes, introduced by the Shixian calendar, is known as . Under this method, the determination of solar terms is similar to the astronomical determination of the special cases of equinox and solstice dates, with different ecliptic longitudes to solve for. One can start with an approximation and then perform a correction using the anomalies and mean motion of the sun. The JPL Horizons On-Line Ephemeris System may be used to query for exact times of solar terms.

The older method, called the , simply divides the tropical year into 24 equal parts.

See also

• Chinese calendar

• Korean calendar

• Japanese calendar

• Vietnamese calendar

• Wheel of the Year

References

External links

• 24 Solar Terms calculator

• 24 Solar Terms – Hong Kong Observatory

• Twenty Four Solar Terms and Their Customs in China

• Chinese calendar with upcoming solar terms in current year

• 24 Sekki – NAOJ > ECO > Glossary >

• Twenty-four Solar Terms and meaning

• Date and time of Solar Terms

• The 24 Solar Terms